Tar spot of corn has been confirmed in Holt and Marion Counties along with 6 Iowa counties.

This is the earliest we have seen tar spot in Missouri. The disease was present at a very low incidence and severity in both the northwest and northeast regions of Missouri where it was found. Now is the time to scout for the disease and monitor. We want to hold off on those fungicide applications a little longer. Treatments at VT through R3 have shown the most consistent results in research on tar spot management.

Figure 1 Corn leaf with tar spot. Some lesions can be surrounded by brown rings (fish eyes) while others are not.

When scouting, look for individual stroma or black lesions on leaf surfaces (Figure 1). Stroma can be mistaken for insect frass (poop). Sometimes brown circles or "fisheyes" may surround the lesions. Initially, there will be very few lesions. We have a few previous articles with descriptions, and the Crop Protection Network has many resources on tar spot.

As you scout, think through some of the known factors that can increase risks of the disease progressing:

• Was tar spot previously confirmed in this field or neighboring fields? We know the pathogen can survive Missouri winters.
• Is this a corn following corn rotation? Corn is the only known host for tar spot.
• Which corn varieties are most vulnerable? (While that information may not be published; seed dealers may have insights.)

Additionally, please submit suspect samples to the MU Plant Diagnostic Clinic for confirmation and to help us track progression of the disease. Confirmed incidences will appear on the corn IPMPipe website.

The fungal pathogen that causes tar spot, Phyllacora maydis, prefers milder air temperatures. Webster et al. (2023) recently showed that extended periods of cooler minimum air temperatures corresponded to increased risks of tar spot development. Average minimum air temperatures in northern Missouri were cooler in June 2023 when compared to the previous few years (Table 1). This might be one contributor to early onset of the disease.

Table 1 Average Minimum Air Temperatures (°F)^*

^*MU Mesonet Historical Data from June 1 to June 23 retrieved from Corning (Northwest) and Novelty (Northeast) through mesonet.missouri.edu

Over the next 6 to 10 days, air temperatures in northwest and northeast Missouri are anticipated to be warmer than normal. These forecasted air temperatures, low incidences of disease, previous research results on application timing, and drought conditions are all reasons to scout and monitor for tar spot but hold off on fungicide applications a little longer.

None the less, if a person chooses to spray early, there are a couple of things to consider: 1) association of NIS applied to V12-V14 corn and arrested ear development; 2) a second application will likely be needed if the disease continues to spread.

If you are in northeast or northwest Missouri, where we know the pathogen is present, please consider utilizing the free Tar Spotter App, developed by colleagues at the University of Wisconsin, to help guide fungicide applications. This mobile app assumes the pathogen is present, and thus the app is not yet appropriate for counties in the state where we are unsure whether the pathogen is present.

We will discuss tar spot at the MU Pest Management Field Day on July 6th. Additionally, consider subscribing to the MU Crop & Pest Text Alert System to keep up-to-date on the latest.

Webster W, Nicolli C, Allen T, Bish M, Bissonnette K, Check J, Chilvers M, Kleczweski N, Mueller B, Price P, Paul P, Robertson A, Ross T, Schmidt C, Schmidt R, Schmidt T, Shim S, Telenko D, Wise T, Smith D (2023) Uncovering the Environmental Conditions Required for Phyllacora maydis Infection and Tar Spot Development on Corn in the United States for Use as Predictive Models for Future Epidemics. bioRxiv doi: https://doi.org/10.1101/2023.03.18.533264.